The cerebral capillary fraction uptake from brains of rabbits subjected to pO2 saturation or pCO2 tension change showed greater reduction of 3H 2-deoxy-D-glucose uptake in hypoxia than hypercapnia and an increased uptake in hypocapnia when the uptake was calculated as percent of control uptake. The effects of the altered pO2 saturation and pCO2 tension in the capillary fraction were similar to the one obtained in the rabbit brain studied in vivo suggesting that the cerebral glucose uptake may be directly related to the capillary function.